Service observing equipment provide means for monitoring a telephone network in order to determine the quality of service that-is being given to the subscribers served by that network. The end product of the service observation equipment is a report (called a "call record") telling the management and maintenance personnel of such networks: how many calls went through which equipment, the length of time required to perform the various call functions, the number and type of call failures, the location of congestion, flagrant dispositions, missing messages, excessive durations, premature cut offs, peak and overall link loading and the like. From these reports, the telephone company may acquire many advantages ranging from reports on routine maintenance, instant maintenance, planning future expansion, justifying billings and rate hikes, selling additional services and the like.
For traditional telephone networks, the service observation equipment merely attached itself to a call path and kept track of everything pertinent to the control of the network functions occurring on that path. This was possible because voice and all control signals were carried by the same call path and generally within the same signal band width (called "In-Band Signaling").
The network control over the call path begins with a subscriber station going off hook, a return of dial tone, an appearance of dial signals, etc. If the call went outside the calling office, there continued to be local supervision so that the service observation equipment was kept fully informed about all that it needed to know in order to provide its report. Thus, the service observation equipment simply attached itself to and monitored a single path for the duration of a call, "listening" to and recording the events as they occurred (e.g., dial signals, connect, ring back tone, answer, and hang up, etc.). On trunks and other calls, there may have been added signals unique to such calls, but the observation functions were about the same--sit there and "listen" for control signals on the only path involved in and dedicated to the call being observed.
The service given to the subscriber and the observation of service on these traditional networks were more than adequate during the days when telephone switching equipment operated at relatively slow speeds, as compared to modern digital operations. For example, relay, step-by-step, or cross bar systems operated at the millisecond speeds of electromechanical equipment. Nothing useful would have been gained by setting up a plurality of paths for the same call. Therefore, the service observation equipment had only one path to observe per call.
Another consideration is the nature of the service being given to a telephone network subscriber. In the jargon of the trade, prior to approximately the mid-1960's, a telephone network gave only "plain old telephone services" ("POTS"). Beginning sometime around the mid-1960's, the electronic speeds of computers, electronic switching, and the like, opened the door to many new and highly sophisticated features and services. Network engineering began preparing for the death of "POTS" by designing new protocols for establishing calls, and for giving new and novel services via telephone networks, such as video telephones, direct "conversations" between personal computers, calling subscriber identification, interactive networks and the like.
This death of "POTS" and modernization of telephone service coincided with the explosion of computer capabilities where incredible amounts of data could be transmitted over a single channel in very small fractions of time. The result opened a new world where all of the control signals from most networks could be transmitted over a single link or group of links having relatively low cost and high speed data capabilities. A computer could keep track of everything that was going on in the entire network so that the establishment and supervision of calls no longer had to be segregated on an individual call basis.
There is no longer a need to set up the relatively expensive communication path for voice, video, computer, etc. communication only to release it because switching blockage, busy conditions or the like, are encountered along the way. Rather than tie up expensive equipment, the new approach is not to set up the communication path itself until it is certain that everything is available and ready to be switched. This certainty is achieved by various electronic equipment communicating with each other via high speed data links. Then, after everything is found to be ready, a totally separate, exclusive communication path is set up via more expensive communication grade equipment for the individual subscriber's use.
The above-described new approach to telephone switching is known in the U.S. as "Signalling System Seven" and as "CCS #7" in the rest of the world. The System is defined by a protocol or standards for uniquely U.S. problems. The protocol for CCS #7 in the rest of the world is defined by the CCITT, which is the international committee for setting communication standards. For the most part, the protocols for SS #7 is defined by Bellcore. Signaling System Seven and CCS #7 are almost identical, except for differences which are required by existing equipment, local rules and regulations, and the like. Therefore, with minor modifications, the same service observation equipment can be used for both systems.
In general, these two protocols provide for a limited number of high speed, synchronous data links extending between switching transfer points ("STPs") which enable switching points or end offices ("SPs") to communicate directly with each other, independently of the voice or other communication paths used by the subscriber. More particularly, the calling office assembles certain required control, monitoring, and supervision information into data packets, each of which is identified by its own address assigned on a per call basis. The data packet also contains identifications of calling and called subscribers, types of calls, equipment, and other pertinent information relating to the call identified by the packet address or "routing label." A description of a data packet and the manner in which it is transmitted over data links has sometimes been likened to a letter and its journey through the mail. The data packet itself is similar to an envelope which is identified by a sender's address and a recipient's address. The envelope may travel over any of many alternate paths guided by these addresses. In fact, one might predict that an average envelope traveling from, say, New York to Los Angeles will likely pass through Chicago. However, there is no way of ever knowing which path will be followed by any individual envelope.
Converting this analogy to the Signalling System Seven, the data packet corresponds to the envelope. The "point codes" (the points to which and from which the data packets flow) of the data packet correspond to the addresses on the envelope. The sender's or return address is called the "Origination Point Code" ("OPC"). The recipient's address is called the "Destination Point Code" ("DPC"). The various facilities that may handle the data packet while en-route are called "Signal Transfer Points" ("STPs") if at an intermediate location or "Switching Points" ("SPs") if at the end offices.
The facilities which carry the data packets are called "Links." A given "A-Link" carries data packets only to or from the owner (SP) of the A-link. STPs, and thus A-links connecting SPs to STPs, occur in redundant pairs. A pair of STPs are linked by "C-links", which can provide alternate routing between the pair if one of the pair should overflow, go out of service, or have to transfer its load. An "E-Link" may provide an alternative to the A-Links by connecting a particular SP to other STPs. An "F-Link" extends directly between end offices (SPs) to bypass the other links, if necessary.
When all information is assembled into the data packet, it is transmitted over a high speed data link to a distant STP, SP, or other appropriate office. The appropriate STP's or offices communicate with each other, agreeing between themselves on whatever is required in order to set up, supervise, monitor, and release the communication path.
To put these high speed operations of the STP's, data links and communication paths into perspective, a large metropolitan area, such as New York City, may require only a total of four STP's and eight high speed data links to establish, supervise, and monitor all calls extended over 65,000 trunks. In New York City, for example, the service observation equipment monitors in the order of 700,000 calls per day on up to 6500 simultaneous "CIC's" (circuit Identification Codes) of the 65,000 trunks in the area. While all 6500 CIC's could occur simultaneously, that is an extremely small probability. At least theoretically all 65,000 trunks could be "busy" simultaneously, but that would not affect the high speed data links, for they are used only while sending data packets for "set up". These statistics are even more startling when one realizes that the high speed data links are usually operated at only 10-15% of capacity (loading) for reliability and safety of communication. Indeed, they are engineered not to exceed 40% loading at maximum saturation.
One can imagine that if a pair of the four New York STP's should go into a failure condition, not only would one half of the city calls fail, but all other telephone equipment across the nation which is trying to communicate with the failed STP's would also fail. The tie up could extend to associated equipment on a worldwide basis. To put the difficulty of and need for service observation equipment into better perspective, when one pair of the high speed data links recently went out of operation due to a software glitch, there was a mass communication failure which paralyzed the entire New York City, much of the telephone equipment in the northeast United States, and some equipment in other parts of the world.
The problem of service observation is further complicated because the STP's use any available data link whenever it is necessary for equipment to communicate with other equipment. The pertinent call information is routed by the addresses in the data packets, and not by the link or path that delivers the data. Thus, all eight data links (in the above example) could be involved at one time or another during the course of setting up a single call.
Moreover, telephone equipment is almost never completely replaced. Somewhere, at least a bit of almost every kind of telephone equipment installed over the last half century or so is still operating. Therefore, in the midst of monitoring and observing all of this new high speed data for electronically controlled equipment, the calls often run into traditional offices giving POTS service without the observation equipment necessarily knowing in advance what kind of equipment will be the next to return a pertinent signal.